The use of micromachined inertial sensors has entered a continual growth phase, especially in the fields of aeronautics, automobiles and robotics, and in yet other fields, thanks to the fact that at the present time these microsensors combine their robustness with advantages associated with their extremely small size. To this should be added the fact that these microsensors can be fabricated collectively (the fabrication operations are carried out on entire wafers comprising numerous sensors that are subsequently divided into individual sensors), which makes the fabrication cost competitive with the prior devices.
Such sensors, produced on silicon wafers, are already known, the fabrication comprising especially boron diffusion and dry anisotropic etching operations for defining precise dimensions of the elements of the structure. Overall, the structure is planar, lying in the plane of the silicon substrate in which it is etched.
The structure of a gyroscope thus produced typically comprises two moving masses that are excited in vibration and connected as a tuning fork, that is to say the two masses are connected to a central coupling structure that transfers the vibration energy from the first mass to the second mass, and vice versa.
The masses are excited into vibration in the plane of the structure by electrostatic forces applied by means of combs of interdigitated electrodes. This vibration in the plane of the structure is exerted perpendicular to an axis called the “sensitive axis” of the gyroscope, which is an axis of symmetry of the tuning-fork structure. When the gyroscope rotates at a certain angular velocity about its sensitive axis, the composition of the force vibration together with the angular rotation vector generates, by the Coriolis effect, forces that set the moving masses into natural vibration perpendicular to the plane of the structure.
The vibration perpendicular to the plane is detected capacitively by electrodes placed above the moving masses. The electrical signals that result therefrom are used to deduce from them a value of the angular velocity about the sensitive axis.
In the prior art, relatively complex structures have been proposed.
To obtain a gyroscope with sufficient sensitivity, that is to say with an ability to detect low rotation velocities, it is necessary for the amplitude of the excited vibration in the plane of the moving masses to be large (compared with the dimensions of the flexure arms that support them). However, a high vibration amplitude generates substantial elastic forces and therefore substantial deformation potential energies. This has the immediate effect of causing nonlinear phenomena to appear in the dynamic deformation characteristics of the structure. The mechanical resonance frequency of the system becomes highly dependent on the amplitude of the movement, a situation that is difficult to accept.
The existing structures generally comprise a moving mass supported by flexure arms clamped to the moving mass, each arm being, on the other side, itself supported (again clamped) in the coupling structure with the other moving mass. In such structures, it has also been sought to attenuate the flexure arm deformation effects by establishing articulated links rather than clamped links between the flexure arms and the moving mass or between the flexure arms and the inter-mass coupling structure. However, these improvements lower the quality of the mechanical coupling between moving masses, degrading the Q of the mechanical resonance of the excited structure. In particular, this poor coupling results in an insufficient frequency difference (of the order of a few tens of hertz) between the useful vibration modes (in phase opposition) of the masses, and the parasitic (in-phase) vibration modes of the same masses.
Hybrid structures have also been proposed (U.S. Pat. No. 5,635,638) in which the moving masses are supported by flexure arms acting both as flexure arms for supporting the moving mass and for defining (via their stiffness) the natural resonant frequency of the masses and also acting as a coupling structure for coupling with the other moving mass in order to promote the anti-phase movement of the two masses. However, the drawback of these structures is the difficult design, owing to the twin roles of these hybrid arms. There is not sufficient independence between the two flexible suspension functions (parallel and perpendicular to the plane of the masses) and the role of coupling mechanical energy between the masses. As a result, there is a high risk of deformation of the structure during vibrations. Here again, there is a risk of nonlinear phenomena, and it is particularly difficult to choose the flexibility characteristics of the arms in order to achieve the desired performance criteria.